1. Field of the Invention
The present invention relates to drying semiconductor structures. More particularly, the present invention relates to the minimization of water spotting and of oxide growth that are experienced on semiconductor structures during chemical treatment steps, deionized (DI) water rinse steps, and drying steps. Examples of minimization of water spotting and of oxide growth are given for an HF rinse process in which the steps of DI water rinsing and drying are combined.
2. State of the Art
Producing a substantially impurity-free semiconductor structure is an ongoing challenge during fabrication of operable integrated circuits and other microelectronic devices. During fabrication, several operations are completed with a chemical treatment such as a polysilicon etching, a photoresist stripping, an RCA cleaning, or a buffered oxide etching (BOE). Following a chemical treatment, rinsing the structure to remove treatment chemicals is required.
Following a chemical treatment such as an HF rinse cleaning, a semiconductor structure is typically transferred to a vessel for a DI water rinse and then to a dryer to be dried. With each transfer of the semiconductor structure during fabrication between processing vessels, the possibility of contamination increases and, with that, also the likelihood of lower process yield. In chemical treatments of semiconductor structures that result in exposed hydrophobic surfaces, the possibility of oxidation and particle contamination is high.
During an HF-last rinse of a semiconductor structure with exposed silicon, for example, a significant number of (Si)2═O bonds are changed to Si—H bonds. About ten to twenty percent of the changed bonds, however, are Si—F instead of the preferred Si—H. During the industry standard DI water rinse that follows most chemical treatments, the Si—F bonds in this example are easily washed off, and oxidation of semiconductor materials, such as silicon, occurs while transferring the semiconductor structure from a rinsing vessel to a drying vessel. Oxidation can occur both during the transfer to the drying vessel and while the semiconductor structure resides in the drying vessel prior to drying.
Before the semiconductor device is moved to the next fabrication phase, substantially complete drying must be accomplished because any water that remains on the surface of a semiconductor structure has the potential of interfering with subsequent processing. Drying can be accomplished in spin-rinse dryers (SRDs), Marangoni dryers, axial dryers and others used in the art.
Various drying techniques such as spin drying also cause water spotting. During spin drying, water spotting droplet tracks are left on the hydrophobic faces of the semiconductor structures. These water spotting tracks are formed from slight impurities contained in the DI water droplets. Water spotting is caused due to the hydrophobic nature of the cleaned silicon and other surfaces such as metallization lines.
As DI water is spun off from the hydrophobic face of a semiconductor structure, water droplet size decreases. Any portion of the DI water droplet that is not pure water is attracted to the hydrophobic surfaces of the semiconductor structure, while the water portion is repelled. Because the water droplets become exceedingly small, dissolved impurities are more strongly attracted to the hydrophobic surfaces of the semiconductor structure than they are to remaining in solution within the water droplet.
The Marangoni drying technique reduces water spotting that is incident to spin drying. In the Marangoni drying technique, a chemically treated semiconductor structure is DI water rinsed, transferred to the Marangoni dryer, immersed in a DI water bath, and drawn through an isopropyl alcohol (IPA) layer that rests on the surface of the DI water bath. In the Marangoni technique, the forces that attract impurities to exposed hydrophobic semiconductor structure surfaces are balanced by the bulk of the water in the relatively pure DI water bath that tends to keep the impurities in solution. When employing the Marangoni drying technique, the DI water and its impurities are entrained beneath the water-IPA interface while the semiconductor structure is drawn through the interface.
Marangoni drying reduces water spotting, but it does not ameliorate oxidative contamination that occurs upon a semiconductor structure during transfer of the semiconductor structure from the DI water rinsing vessel to the dryer. Thus, elimination of the deleterious effects of water spotting may be overshadowed by contamination of the semiconductor structure experienced simply during transfer from one vessel to another.
Because a Marangoni dryer may have moving parts, the function of which is to draw a wafer boat out of a DI water bath, the possibility of particulate contamination arises, which contamination is caused by abrasion of surfaces on the moving parts.
What is needed is a method of rinsing and drying semiconductor structures, particularly hydrophobic semiconductor structures, in such a way as to substantially eliminate oxide contamination incident to semiconductor structure transfer from vessel to vessel and by water spotting incident to spin drying. What is also needed is a device that will accomplish the inventive method while simplifying the Marangoni technique and equipment.